One of the primary influences on tumor radiocurability is the presence of hypoxic but clonogenic tumor cells which are 2.5 to 3 times more resistant to radiation than aerobic cells. These cells occur as a result of the abnormal microcirculation which exists in tumors. The impact of these cells is partially reduced by the process of reoxygenation which occurs with fractionated radiotherapy. Hyperthermia can also influence the relative importance of these cells in two ways: 1) By reducing their radioresistance 2) By destroying some of the tumor microcirculatory bed resulting in infarction. Proper use of hyperthermia with radiation will be dependent upon the relative effects of these modalities on tumor and adjacent normal tissue microcirculation. A transparent access window Algire chamber developed at the University of Arizona with implanted tumor and normal tissues will permit direct visualization of normal and tumor microvasculature. It will allow for quantitative measurement of microvascular branching patterns, tissue perfusion based on single capillary blood flows. Local values of pO2 and pH will be made using microelectrodes. These measurements will be carried out for different time-temperature combinations and heat fractionations, radiation alone and combinations of heat and radiation. This information will lead to an understanding of how to optimize the combined use of heat and radiation leading to maximum tumor and minimum normal tissue effect.